When images are captured by a camera, there are a number of factors that can adversely affect the quality of the resulting image. One important factor is the sharpness of the image. There are several attributes of the imaging system that affect the sharpness of the image, such as the quality of the lens and the MTF of the image capture medium. But beyond these fundamental characteristics, there are other image-dependent factors that can seriously degrade the sharpness, and therefore the image quality, of the image. These factors include overall lens defocus (e.g., due to attempting to capture an image of an object that is too close to the camera), defocus of the main subject (e.g., due to the camera auto-focusing on the wrong object in the scene), insufficient depth-of-field to capture the range of important object distances, fogging of the lens element due to condensation of water vapor, and smearing due to motion of the camera and/or objects in the scene.
In conventional photography using photographic film, it is usually not possible to determine whether the captured image has acceptable sharpness until the film is processed and printed. With the advent of digital cameras, it is possible to preview the image at the time of capture. In theory, this allows the photographer to assess the quality of the image, and, if necessary, capture another image to correct image quality problems. However, the quality of the preview displays used on most digital cameras is insufficient to adequately evaluate the image quality in many cases. As a result, the photographer may not realize that the quality of an image is poor until after the image has been printed or previewed at a later time on a high-resolution display. As a result, the photographer may miss any opportunity to capture an improved image, or may be dissatisfied that time/money was wasted in making a print of a poor quality image. Therefore, there is a need for a means to automatically assess the sharpness of a digital image at the time that the image is captured.
There are examples of prior art in this field. Some cameras offer a warning signal if camera shake is likely to occur due to an excessively long exposure time. Tomita discloses (U.S. patent application Publication 2004/0018011 A1) a vibration correcting optical device comprised of a vibration detection unit that detects a vibration of the vibration correcting optical device and outputs a vibration detection signal corresponding to the vibration. Wheeler, et al. disclose (U.S. patent application Publication 2003/0095197 A1) a method of using image metadata to predict camera shake. A significant problem with these arrangements is that they do not directly measure the level of blur in the captured image itself, but rather attempt to predict whether the image might be blurred based on other pieces of information. As a result, these methods are only effective at identifying certain sources of image blur (e.g., blur due to camera shake).